Krugman got me thinking…

Paul Krugman posted a blog entry about the tired old canard that life expectancy was much lower when Social Security was implemented, so increasing life expectancy is bankrupting the system. Of course, the drastic increase in life expectancy, from 68 to 78 is improvements in saving the lives of children. This gets at a fundamental aspect of life expectancy calculation.

When we talk about life expectancy, we’re usually referring to life expectancy at birth. There are many reasons we use this statistic and many reasons it’s not a reliable indicator for people of various ages. Prior to the development of modern, evidence-based medicine, mortality rates during various stages of life were quite high. All deaths at all ages are incorporated into the calculation of life expectancy at birth. In a society where there is no modern medicine, many infants don’t survive their first year. If a child survives that year, their life expectancy increases because we’re calculating the statistic from a different sample, namely all of the other people who lived at least one year. Similarly, childhood diseases kill. A child who survives to puberty has a greater life expectancy from that point on for the same reason he or she gained increased life expectancy for surviving the first year of life. The kids who didn’t make it (and there are a lot of them) are no longer included in the calculation. Here’s where it gets really interesting. After puberty males and females are subject to different effects on life expectancy.

Females have their fertile years ahead. Before modern gynecological and obstetric care, there was a huge risk to having children. Child birth is dangerous, and many women died (still do in developing countries) during the process. Making it through their childbearing years into menopause again increased the life expectancy of women. Males don’t suffer the high death rate caused by child birth, but they are the ones who have historically fought in wars. Surviving long enough to no longer be considered for military service increases the life expectancy of men.

So, if you make it to your middle years, there’s a good chance you’ll live to be 65 or so. That’s why even in countries where the life expectancy is reported as very low, there are elderly people roaming about. They’re the ones who survived the various dangerous stages and therefore will probably live to be “old”.